1. Field of the Invention
The present invention relates generally to improvements in hard disk drive servo methods and, more particularly, but not by way of limitation to improvements in methods for moving transducer heads across disks of hard disk drives.
2. Brief Description of the Prior Art
Computer programs and data generated by a computer are often magnetically stored in a hard disk drive; that is, a device having one or more rotating aluminum disks that have magnetizable coatings so that transducer heads, through which a current can be passed to produce a magnetic field, that fly over the disk can magnetize successive cells of tracks defined in the disk coatings. The data, or programs, can be read at a later time by using the transducer head to detect changes in the magnetic field along the surfaces of the disks resulting from the magnetization of the coatings and movement of the disk surface by the transducer heads.
The major advantages of the hard disk drive are that they are capable of economically storing large amounts of data which can be accessed very quickly. For example, a hard disk drive capable of storing an amount of data of the order of a gigabyte can effect the storage of a block of data, or retrieve a block, in a time that is measured in milliseconds. Because of these advantages, hard disk drives are being increasingly selected as a long term data storage device in computer systems.
The advantages of the hard disk drive are achieved by storing the data on closely spaced, concentric data tracks defined on the disks and providing the drive with a servo system that accurately positions the transducer heads with respect to these tracks. To this end, the transducer heads are mounted on an actuator via arms that extend along the disk surfaces and are movable in response to currents passed through a coil on the actuator, such coil being immersed in a magnetic field so that the current gives rise to a force on the coil in accordance with the familiar Lorentz relation. Servo data is written to one or more of the disks to be read during track following %o provide a position error signal that is proportional to the misalignment of the transducer head with the track being followed and a control signal is generated from the a position error signal and transmitted to a power, transconductance amplifier that passes the current through the actuator coil to form a closed loop servo system that corrects such misalignment. The use of a closed loop servo system enables the data tracks to be closely spaced to maximize the data storage capacity of the drive.
In order to read or write a selected block of data, the data tracks are divided into sectors which are assigned to specific blocks and control circuitry is provided to address the sector and track at which the data is located. Thus, the transducer head can be moved to the track on which any selected block of data is located or on which it is to be written by addressing the track and operating the servo system to carry out a seek to that track. In the seek operation, the servo system receives the address of the destination track and generates control signals that cause the transducer heads to initially accelerate toward the destination track and subsequently decelerate as it nears such track.
A common way of controlling the seek to cause the movement of the transducer head to the destination track is to develop a velocity profile that provides the velocity the transducer head should have at varying distances from the destination track and, at each of a succession of tracks terminating with the destination track, providing a control signal to the power amplifier that is directly proportional to the difference between the profile velocity and the actual velocity of the transducer head. The profile is shaped with respect to the number of tracks remaining in a seek to cause the transducer head to initially accelerate toward the destination track and subsequently decelerate to the track. In long seeks, these stages of the seek may be separated by a stage in which the transducer head traverses a series of tracks at a maximum speed that is selected on the basis of any of a number of criteria used by the manufacturer of the disk drive. For example, the maximum speed may be chosen to be the maximum speed the transducer head can attain with the power supply that is used to operate the servo system.
The use of a velocity profile that can be developed with respect to any selected servo system operating criteria can be used to minimize the time required for the seek to occur and still reach the destination track with a speed that is neither too large nor too small to effectuate a rapid settling of the transducer head on the destination track at the end of the seek. Specifically, since the control signal is proportional to the difference between the profile velocity and the actual velocity, the transducer head can be caused to rapidly accelerate at the beginning of the seek by providing a profile that calls for large velocities at the beginning of the seek and then rapidly tapering the profile to zero as the destination track is reached.
A problem that has arisen with respect to the use of a velocity profile to effect seeks to a destination track is that noise is generated during the seek and, moreover, the intensity of the noise is determined by the form of the profile. In general, the greater the acceleration called for by the profile, the noisier the disk drive during the seek. Such noise can be very distracting to the user of a computer so that it is desirable that it be limited in a way that is consistent with the need for limitation of the time required for making seeks.
In the past, this compromise has been difficult to implement because of the manner in which the velocity profile is developed. In general, the profile is developed by requiring that the coil current satisfy a selected current versus time relationship during deceleration assuming that the transducer head begins deceleration from the maximum velocity that will be attained during the seek. Since the force exerted on the actuator is proportional to the coil current, a knowledge of the velocity at the start of the deceleration and the current profile permits the velocity and location of the head to be determined at any time during deceleration. Thus, the velocity profile, as a function of tracks remaining in the seek, can be found for the deceleration of the head by eliminating the time in equations expressing the velocity and location. Remaining portions of the profile, that is, portions of the profile corresponding to acceleration of the transducer head and coasting after the maximum head velocity has been attained, are generated by just setting the profile velocity equal to the maximum velocity. It will be noted that only the deceleration of the transducer head is utilized in developing the profile. While it would, in principle, be possible to develop the profile for the entire seek, in practice such development would be very difficult because of the dependence of the profile development method on the head beginning the deceleration with a known speed.
The noise arises from large differences between the profile velocity and the actual velocity that will give rise to large, rapidly changing currents through the actuator coil during acceleration and deceleration of the transducer head. The currents give rise to the forces that accelerate the transducer head so that passing large, rapidly changing currents through the actuator coil has the same effect that striking the actuator with a mallet would have; that is, a large impulse is delivered to the actuator. Moreover, the magnets that are mounted on the case of the disk drive to provide the magnetic field in which the actuator coil is immersed experience the same impulse in accordance with Newton's third law of motion. The effect of these impulses is to excite vibrational modes of the case, the actuator and other portions of the drive. While this effect can be minimized for the deceleration of the actuator by appropriate shaping of the current versus time relation during deceleration from which the velocity profile is developed, the effect is not easily treated for acceleration because of the difficulty in modeling the entire seek that has been noted above.
Because of this noise problem, several suggestions have been made concerning the manner in which the seek is controlled. Thus, for example, it has been suggested that the seek be performed by requiring the acceleration to be a selected function of time for the entire seek. The difficulty with this approach is that the Lorentz force is not the only force on the actuator; for example, the actuator is also subjected to windage forces caused by the swirling of air by the disk rotation and the support of transducer heads above the disk surfaces by the air current so provided. Similarly, flex forces are exerted on the actuator by plastic strips that carry electrical conductors between the drive case and the actuator. In general, these forces depend upon the location of the destination track and the initial track from which a seek is commenced. Unless carefully compensated, these forces can cause the seek to terminate at an unsuitable location with respect to the destination track.
It has also been suggested that the gains of amplifiers in the servo circuit be adjusted during the acceleration stage of a seek in relation to the distance remaining in the seek so that such amplifiers have a very low gain at the start of a seek and a higher gain as the seek nears completion. This difficulty with this approach is that it shifts the problem of developing a complete seek profile to one of developing a gain profile. Thus, nothing is gained. Moreover, since the amplifier gains are very low at the beginning of the seek and are increased in relation to the distance remaining in the seek, the Lorentz force on the actuator coil will be very small at the beginning of a seek and can be balanced by other forces on the actuator. In this case, the actuator will hang up on the track from which the seek is initiated.
As a result of the aforementioned problems, the elimination of noise during seeks in a hard disk drive has, prior to the present invention, proven to be a difficult problem. While it can be solved, the solutions that have been suggested in the past give rise to new problems that make the solution as difficult to implement as profiling the entire seek.